Data Sheet ADCMP370/ADCMP371
APPLICATIONS
BASIC COMPARATOR
In its most basic configuration, a comparator can be used to
convert an analog input signal to a digital output signal. The
analog signal on IN+ is compared to the voltage on IN−, and
the voltage at OUT is either high or low, depending on whether
IN+ is at a higher or lower potential than IN−, respectively.
The ADCMP370 and ADCMP371 have different digital output
structures. The ADCMP370 has an open-drain output stage
that requires an external resistor to pull OUT to the logic high
voltage level when the output transistor is switched off. This
voltage level can be as high as 22 V. The same 22 V tolerance
also applies to the inputs of the comparators. The pull-up
resistor should be large enough to avoid excessive power
dissipation but small enough to switch logic levels reasonably
quickly when the comparator output is connected to other
digital circuitry. A suitable value is between 1 kΩ and 10 kΩ.
The ADCMP371 has a push-pull output stage, which has an
internal PMOS pull-up and, therefore, does not require an
external resistor. Faster switching speeds between low and high
rails are possible, but the logic high level is limited to V
CC
.
ADM331
OUT
V+
V
REF
t
V
OUT
0V
ADCMP370
V
CC
V+
V
IN
04745-017
V
REF
V
IN
Figure 18. Basic Comparator and Input and Output Signals
ADDING HYSTERESIS
To prevent oscillations at the output caused by noise or slowly
moving signals passing the switching threshold, positive feedback
can be used to add hysteresis to the differential input.
For the noninverting configuration, shown in Figure 19, two
resistors are used to create different switching thresholds,
depending on whether the input signal is increasing or
decreasing in magnitude. When the input voltage is increasing,
the threshold is above V
REF
, and when it is decreasing, the
threshold is below V
REF
.
The upper input threshold level is given by
( )
R2
R2R1V
V
REF
IN_HI
+
=
The lower input threshold level is given by
( )
R2
R1VR2R1V
V
CC
REF
IN_LO
−+
=
The hysteresis is the difference between these voltage levels
In the example in Figure 19, Resistor R1 and Resistor R2 are
chosen to give 1 V hysteresis about the reference of 2.5 V, with
V
CC
= 5 V.
ADM331
AD
CM
P371
V
CC
= 5V
R2 = 1MΩ
R1 = 200kΩ
V
REF
= 2.5V
IN+
IN–
V
IN
V
OUT
V
IN
V
IN_LO
= 2V
V
IN_HI
= 3V
04745-018
OUT
Figure 19. Noninverting ADCMP371 Comparator Configuration with
Hysteresis
With the inverting configuration, the upper and lower
switching thresholds are
( )
R2R3R1
R2V
V
CC
IN_HI
+
=
||
( )
)()1()(
1
R3R2RR2R3R1
R3RR2V
V
CC
IN_HI
×+×+×
+×
=
( )
( )
R3R2R1
R3R2V
V
CC
IN_LO
||
||
+
=
)3()()3(
32
RR2R1R2RR1
RRV
V
CC
IN_LO
×+×+×
××
=
Rev. D | Page 9 of 12
